1. Field of the Invention
The present invention relates to a sheet feeder used, for example, in an image forming apparatus, such as a laser beam printer or an electronic copying machine, and an image forming apparatus which forms an image on a sheet fed from the sheet feeder.
1. Description of the Related Art
A conventional sheet feeder is constructed as shown in FIG. 11, which is a sectional view of a laser printing type image forming apparatus 1. A plurality of sheets P, loaded on a sheet feed tray 2, are successively fed one sheet at a time by a sheet feed roller 3. Reference numeral 4 denotes a pair of conveyor rollers that convey a sheet P fed by the roller 3 toward a transfer roller 5. Reference numeral 6 denotes a process cartridge which integrally incorporates a photosensitive member 7, such as an electronic photosensitive drum, which is an image carrying member, and image forming means, such as a developing means. Reference numeral 8 denotes a laser scanning optical system for forming an electrostatic latent image on the photosensitive member 7. A toner image transferred onto the sheet P by the transfer roller 5 is fixed onto the sheet P by heating with a fixing device 9, after which the sheet P with the fixed image is discharged onto a discharge tray 11 by means of a pair of discharge rollers 10.
Various proposals have been made to prevent displacement of the printed image from occurring when the sheet P on the sheet feed tray 2 is obliquely fed as it moves to the transfer roller 5. It may be prevented by:
(1) Providing a side regulating member 12 for sandwiching both sides of the sheet on the sheet feed tray 2, or PA1 (2) Stopping the rotation of the pair of conveyor rollers 4 until the leading end of the sheet P reaches the nip portion of the pair of conveyor rollers 4, and forming a loop between the sheet feed roller 3 and the pair of conveyor rollers 4 in order to properly register the sheet, thus eliminating oblique movement.
However, proposal (1) has the problem that the sheets on the sheet feed tray 2 tend to tilt, since the side regulating member 12 is provided only on the sheet feed tray 2. In addition, the side regulating member 12 cannot effectively prevent oblique movement after the trailing end of the sheet has left the side regulating member 12. Proposal (2) has the problem that a clutch mechanism, formed by an electromagnetic clutch or the like, must be provided at the conveyor roller driving section in order to properly register the sheets by the pair of conveyor rollers 4, thereby increasing manufacturing costs.